nspawn: register a scope for the unit if --register=no is specified (#6166)
Previously, only when --register=yes was set (the default) the invoked
container would get its own scope, created by machined on behalf of
nspawn. With this change if --register=no is set nspawn will still get
its own scope (which is a good thing, so that --slice= and --property=
take effect), but this is not done through machined but by registering a
scope unit directly in PID 1.
Summary:
--register=yes → allocate a new scope through machined (the default)
--register=yes --keep-unit → use the unit we are already running in an register with machined
--register=no → allocate a new scope directly, but no machined
--register=no --keep-unit → do not allocate nor register anything
Fixes: #5823